A solid oxide fuel cell consists of a solid electrolyte sandwiched between a porous air electrode and a porous fuel electrode. A fuel, such as a mixture of methane and water, is passed over the porous fuel electrode and oxygen is passed over the porous air electrode. Oxygen ions migrate through the electrolyte to the fuel electrode where they react with the fuel, generating heat and electricity.
The porous fuel electrode in one embodiment, is made of nickel particles held in place by an oxygen-ion conducting metal oxide skeletal coating. It has been discovered that, after many hours in operation, the nickel particles in the fuel electrode next to the electrolyte/fuel electrode interface are gradually removed or transferred, resulting in a deterioration of cell performance.
Accorsi and Bergmann, in an article in the Journal of the Electro Chemical Society, Volume 127, No. 4, pages 805-811 (April, 1980), observed this phenomenon of metal migration, as seen in their electrolysis experiments. They attempted to eliminate this behavior by alloying the nickel with titanium to reduce diffusion of the nickel. However, they reported that no significant improvement in performance of their electrolysis device occurred.
In another area, Isenberg, in U.S. Pat. No. 4,702,971, and U.S. patent application Ser. No. 072,834, filed on July 13, 1987, now U.S. Pat. No. 4,812,329 in an attempt to improve performance and sulfur tolerance of fuel electrodes of solid oxide fuel cells, coated the nickel particles and zirconia skeleton of the fuel electrode with a complete or partial, thin, single coating of an ionic-electronic material selected from doped or undoped ceria and doped or undoped urania. Coating cracking and flaking occurs with such coatings, however, allowing nickel exposure during fuel cell operation. Such nickel exposure was not seen as a problem in that process, in fact, flaking was seen as adding more active sits to the electrode structure.